Dice onion and cube the potato, then saute them in oil over medium heat.,Once they have softened (about 4 minutes if you keep them covered, stirring occasionally) add the defrosted beans and vegetable stock (chicken stock can be used instead, if necessary) then simmer for 15-20 minutes, until beans are soft.,Turn off heat and pour into a food processor or blender, and puree.,______,Heat the pureed soup another few minutes without boiling it, then serve!,Add salt and pepper to taste.